Desulfurization of olefinic naphtha



Patented Aug. 26, 1952 DESULFURIZATION F OLEFINIC APHT A John F. Deters, Valparaiso, Arthur P. Lien, Ha inmond, and Harold Shalit, East Chicago,'lnd.,

assignors to Standard Oil Company, Chicago, 111., a corporation of Indiana '1 No Drawing. Application November, 29, 1949, a Serial No. 130,068.

4Claims. (01. 196-24) This invention relates to an improved method of desulfurizing highly olefinic naphthas and it pertains more particularly to the desulfurization of coke still naphtha by extraction with dimethyl formamide containing less than per cent of a ter, the extract'being hydrodesulfurized for blending with the raflinate. g i An object of the invention is to providean improved method and means for removing sulfur compounds from highly olefinic naphthas without materially altering the olefin to paraffin ratics and without suffering large treating losses. A particular object; is to desulfurize a naphtha in which the olefinic content substantially equals or exceeds the parafiinic content, an outstanding example of such naphtha being coke still naphtha. A further object of the invention is to convert a high sulfur olefinic naphtha into low sulfur olefinic gasoline of a high anti-knock value with minimum treating losses andv with a minimum amount of olefins subjected to hydrofining whenthatprocess isemployed for converting sulfur compoundsto HzS for facilitating their removal from extract material.

Dirnethyl formamide isknown to be useful in the. extraction of hydrocarbon oils (U; S. 2,166,140) and when applied to heavy oils, it may contain water throughout a wide operating range (U. S. 2 ,183,852) In our invention, the dimethyl formamide is employed for extracting a particular type of stock, namely a high sulfur olefinic naphtha and it performs a function radically different fromthat which it performed in prior processes'. We have discovered thatwhen dimethyl formamide is employed as an extractant for high sulfur olefinic naphtha, it not only effects separation. between aromatic and paraffignic hydro carbons, but it serves the additional function of concentrating the sulfur in the extract phase whilleaving the bulk of the olefins in the rafiinate phase. This remarkable and unexpected property of dimethyl formamide makes it possible to concentrate sulfur from a high sulfur olefinic naphtha and the relatively small volume of extract contains only a minor amount of the olefins. The extract can then be hydrodesulfurized in the absence of raifinate and the hydrodesulfurized extract blended with the rafilnate to produce aremarkably high yield of stable olefinic gasoline of high octane number.

The invention is applicable to high sulfur olefinic naphthas generally, 1. e. to naphthas in which the olefin to paraffin ratio is in the range of about 3:1 to 1:3 and is particularly advantageous for charging stocks in which the olefinic content exceeds the paraffinic content. By'fhlgh sulfur, we mean a sulfur content whichis at least about .25 per cent and which is usually in the range of about .5 to 1 per cent or more.- "By naphtha, we mean a normally liquid hydrocarbon fraction boiling below 450 F. and usually boiling almost entirely within the range of"100 to 406 F. An outstanding example of a high sulfur olefinic naphtha is the naphtha produced by the coking of high sulfur reduced crudejin any known manner such, for example, as by continu ously introducing heated charging stockint'o a coke drum at about atmospheric pressure to 50 pounds'per square inch at a temperature in the range of about 850 to 950' F,, andcontinuingthe operation until coke solidifies inthe drum, the vaporous products from the drum being 'frac tionated into a coke still naphtha fraction and one or more heavier fractions. While the invention may be applicable to high sulfur olefinic naphthas produced by thermal cracking or by catalytic cracking, its outstanding advantage is in the refining of coke still naphtha because this particular high sulfur olefinic naphtha is the most difficult of all to desulfurize or otherwise refine by conventional methods.

Coke still naphtha has an olefin to paraffin rano of approximately 2 1,-and contains from about .25 to 4 per cent sulfur by weight,'-a"representative stream containing for example about lper cent sulfur. The bromine number of the olefinic components of a representative coke still naphtha may be about to but there is a bromine absorption due to sulfur corresponding to about 5 bromine numbersper weight per cent sulfur. A correlation of bromine numbers'and sulfur content in original charge, raflinate and extract, respectively, can thus'be employed to determine the extent to which olefins are selectively removed from paraflins in an extraction process. a

Our extractant is a di-aliphatic formamide, such for example'as dim'ethyl formamide,'methyl ethyl formamide, di-ethyl formamide, etc. "Di methyl formamide'is a preferred solvent, and al though it boilsat 153 C.,' which is within the boiling range of the olefinic naphtha,:itmay be readily separated from raflinate and extract by simple water washing and then separatedfrom water by simple distillation since it doesnot form azeotropes orconstant boiling mixtures with water. Such a solvent i heat stable, but will not polymerize orform tars when heated, is non-- corrosive and is completely miscible Withwater. In addition, its density of 0.953 gm./ml. issuch that it is easily separated from petroleum stocks;

For optimum results, the dimethyl formamide should contain a small amount of water which should be less than 25 per cent and usually in the range of about 1 to 10 per cent, or approximately 2 per cent when the solvent to oil ratio is of the order of 1:1, larger water contents-being permissible with, higher solvent to oil ratios. The Water is most effectively utilized when introduced into the extract portion of a countercurrent exsystem, or by any other extraction method known to those skilled in the art. 1

The extraction can best be carried out with a solvent to charging stock volume ratio in the range of- .3z1 to 3:1, the preferred range being about .5:1 to 1:1. The extraction may be effected atordinary room temperature or within the range ofiabout C. to-50 C. The pressure should be such as to keep all material in liquid phase. .As a specific example of the invention, the following data were obtained when a coke still naphtha (produced by the coking of West Texas gas oil) ofabout 81 brominenumber and-containing about .9 weight per cent sulfur was extracted at room temperature with dimethyl formamide containing 2per cent by volume of water, and employing a solvent to oil ratio of 1:1, the extraction having been effected in the countercurrent column packed with glass beads.

Ratio: Weight Percent S/Oorr. Br. No.

Gorrooted Br. i o.

Br. No.

Weight percent S the-above data it will be noted that the .climethyl formamide was remarkably effective in concentrating the sulfur in the extract phase without preferentially extracting olefins to any appreciable extent. The bromine number of they extract when corrected for its sulfur content was of substantially the same order of ,magnitude as the original charging stock. The-sulfur to olefin ratio in the extract (on the basis of corrected bromine number) was twelve times that of the rafiinate, thus indicating the remarkable effectiveness of the solvent for concentrating sulfur in the extract without appreciably concentrating olefins so that when the extract is subjected to hydrodesulfurization, only a minor pro-portion of the olefins present in the original. charging stock will be subjected to hydrogenation conditions which might lead to thei saturation- 'In another example of our invention, the following data were obtained when a coke still naphthax(from the coking of mixed Mid-Continent and West Texas gas oils) of about .4 weight per cent-'sulfurwas extracted at room temperature with dimethyl: formamide containing the indicated amounts of Water with countercurrent extraction procedure employing a solvent to oil ratioin each case of about .8 1:

W ht v it erg eig Percent meted Percent Percent S No. Br. 8/00 Br. No.

Charge 100 I 4 7 '50 48 008 No water:

Rafilnate 43 03 47 47 0006 Extract 7 57 61 55 52 012 3% water in sol vent:

Ralfinate 77 10 51 50 O02 Extract .1 23 1. ()9 52 47 023 5% water insolvent:

Rafiinate 83 14 48 47 003 Ext1act 17 1.22 51 027 The above data indicate that with no water the rafiin'ate is of remarkably low sulfur content, but produced at a yield of less than volume per cent. With 3 per cent by volume of water in the solvent, the rafiinate yield is almost doubled and its sulfur content is only .10 weight per cent. With 5 weight percent of water in the solvent, the railinate yield is still higher, but its sulfur content has increased to .14 weight per-cent. Thus it appears to be markedly advantageous to employ a water content of about 1 per cent to 5 per cent when the water is employed with introduced solvent and when thesolvent'to oil ratio is approximately 0.8:1. V i i The sulfur to olefin'ratio' (as indicated by Weight per cent sulfur divided by corrected bromine number) ShOWs the remarkableextent to which sulfur is concentrated in the extract to the relative exclusion of olefins. With no water, the extract ratio is about 20 times'the raflinate ratio. .With 2'per cent water, the extract ratio is about 11 times the raifinate ratio. With- 5 per cent water, the extract ratio is about 9 times the rafiinate ratio. The above figures clearly show the remarkable effectiveness'of dimethyl foramide for desulfurizing a high sulfurolefinic naphtha without concentrating olefins in the extract phase. The corrected bromine numbers themselves show that the extract phasefdoes not contain an appreciably higher concentration of olefins than were-present in the original chargfing stock and that with water in the solvent, the extract actually contained a lower proportion of olefins than was contained in the original chargingstock.

It should be understood that increasedconcentrations of water may be used in either of the above examples in order to obtain higher raffinate yields, but in so doing, the extent'of'desulfuri zation will be diminished.

When dimethyl formamide is employed as the extractant, separation of the solvent from extract and rafilnate is effected by simple water washing and the solvent can then be separated from the water by simple distillation. With sum ciently high boiling di-alkyl formamides, the separation of solvent from raffinate and extract may be effected by simple distillation.

The sulfur content of the rafiinates' -produced' by our invention is sufficiently low so that they may be blended in finished gasoline to meet stringent sulfur specifications. The extract which only constitutes a small fraction of the original charging stock, but whichcontains' most of the sulfur compounds, may be'hydrodesulfurized by any known process, such, for example, as hydroforming, hydrofining, autofining, etc. The catalysts employed for this step may be an eighth group'metal or metal sulfide as described in U. S. 2,273,297, but it is preferably a supported sixth group metal compound, such as chromium or molybdenum oxide on alumina as employed in hydroforming (note U. S. 2,388,536) or supported cobalt molybdate as used in hydrofining (note Ind. Eng. Chem, vol. 35, pagesllGO to 1167), or tungsten sulfide-nickel sulfide as described in the paper presented by Eagle and Rudy entitled Application of Cyclic Adsorption Process to Desulfurization of Cracked Naphtha before the American Chemical Society in September 1949. Autofining is similar to hydrofining except that the'hydrogen is supplied by naphthenes added to the charging stock. Since these hydrodesulfurization processes are well-known to those skilled in the art (product yields and product qualities, both alone and blended with initially separated components being likewise known, as described for example in the Eagle and Rudy paper), it is unnecessary to describe the hydrodesulfurization or the yields and properties of finished motor fuel in further detail.

Our invention provides an enormously simpler and less expensive method of treating a high sulfur olefinic naphtha for obtaining a relatively small amount of extract of high sulfur and low olefin content than was heretofore known to the art. Since only the extract requires hydrodesulfurization and the rafl'inate (which contains most of the oleflns) is not subjected to hydrodesulfurization, we minimize hydrodesulfurization expense and avoid saturation of any olefins separated as raffinate. We thus not only obtain higher yields of a finished product blend, but we obtain a finished product blend of remarkably high octane number and at a relatively low cost.

While our invention has been described with respect to specific operating examples, it should be understood that alternative operating conditions and modifications will be apparent to those skilled in the art. Other di-alkyl formamides which are substantially equivalent in extraction properties to dimethyl formamide may be employed instead of dimethyl formamide itself.

We claim:

1. The method of obtaining a low sulfur olefinic gasoline of high anti-knock value from a high sulfur olefinic naphtha which comprises extracting a naphtha having anblefln to paraflln ratio in the range of 3:1 to 1:3 and containing more than .25 per cent sulfur with a di-alkyl formamide under conditions to form an extract phase containing most of the sulfur and a rainnate phase containing most of the olefins, separately removing solvent from each of said phases, hydrodesulfurizing the solvent-free extract phase to eliminate sulfur therefrom and blending the hydrodesuliurized extract with the solvent-free rafiinate.

2. The method of claim 1 which includes the step of employing with the di-alkyl formamide an amount of water in the range of about 1 to 5 per cent by volume based on total solvent.

3. The method of claim 2 wherein the di-alkyl formamide is dimethyl formamide.

4. The method of desulfurizing a high sulfur coke still naphtha having an end point below 450 F. and having an oleflnic content which is higher than its paraifinic content, the sulfur content of said naphtha being in the range of .25 to 4 percent, which method comprises extracting said high sulfur coke still naphtha with dimethyl formamide to produce raflinate and extract phases, employing an amount of water in the range of about 1 to 5 percent by volume based on total solvent at least in the extract phase to in crease rafiinate yield without substantially decreasing desulfurization, separating solvent from the resulting railinate and extract phases, hydrodesulfurizing the solvent-free extract in the absence of olefin separated in the raflinate phase and blending naphtha-boiling-range products of the hydrodesulfurlzing step with the solvent-free railinate to obtain a desulfurized naphtha product rich in olefins.

JOHN F. DETERS.

ARTHUR P. HEN. HAROLD SHALIT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,166,140 Hansley July 18, 1939 2,285,696 Dunn -1. June 9, 1942 

1. THE METHOD OF OBTAINING A LOW SULFUR OLEFINIC GASOLINE OF HIGH ANTI-KNOCK VALUE FROM A HIGH SULFUR OLEFINIC NAPHTHA WHICH COMPRISES EXTRACTING A NAPHTHA HAVING AN OLEFI TO PARAFFIN RATIO IN THE RANGE OF 3:1 TO 1:3 AND CONTAINING MORE THAN .25 PER CENT SULFUR WITH A DI-ALKYL FORMAMIDE UNDER CONDITIONS TO FORM AN EXTRACT PHASE CONTAINING MOST OF THE SULFUR AND A RAFFINNATE PHASE CONTAINING MOST OF THE OLEFINS, SEPARATELY REMOVING SOLVENT FROM EACH OF SAID PHASES, HYDRODESULFURIZING THE SOLVENT-FREE EXTRACT PHASE TO ELIMINATE SULFUR THEREFROM AND BLENDING THE HYDRODESULFURIZED EXTRACT WITH THE SOLVENT-FREE RAFFINATE. 